Cryogenic liquids, such as liquid nitrogen, have been used successfully in a number of low-temperature freezing operations such as food or biological materials freezing. In theory, it was recognized that a number of chemical and pharmaceutical processes also could benefit from cryogenic liquid cooling due to the low temperatures and high driving force afforded by cryogenic liquids. However, use of cryogenic liquids in low-temperature chemical processes has been limited because the low temperature and high driving force can cause the process fluid to freeze. Freezing of the process fluid in chemical operations is undesirable and can be hazardous, especially if the refrigeration is used to control exothermic reactions.
One conventional attempt to avoid the problem of process fluid freezing is to design an oversized shell-and-tube heat exchanger. A heat transfer fluid or reactant is pumped into the tube side under high velocity. A cryogenic liquid, such as liquid nitrogen, is either sprayed or flooded onto the shell side of the heat exchanger. In this type of heat exchanger freezing of the heat transfer fluid will occur as the liquid nitrogen downloads its latent heat of vaporization on the metal surfaces of the tube and shell. When the ice starts to grow and propagate, the heat transfer surface will lose its thermal conductivity. The result is either a rapid loss of heat transfer capacity or a total freezing of the entire contents of the heat exchanger. Upon freezing, the unit must be defrosted before it can be put back to service. For chemical reactions or more generally for heat transfer applications that require a very short batch time (of the order, e.g., of 10-15 minutes), an oversized heat exchanger may provide a solution, as it may remain functional for a limited time before losing its capability to provide effective heat transfer. But if the batch time is significantly longer (e.g. 1 hour) the already oversize heat exchanger needs to be 4-6 times bigger to accomplish the same result (refrigerate the process fluid) without freezing, which prohibitively adds to the cost.
Another conventional approach is to mix the liquid nitrogen with room temperature nitrogen gas to reduce the refrigerant driving force and produce a cryogenic gas at a temperature warmer than -320.degree. F., the condensation temperature for nitrogen at 1 atm pressure since the cryogenic cold gas can be kept as warm as necessary to avoid the freezing problem. In this approach, however, all of the latent heat of vaporization is lost in the mixing process. Furthermore, the nitrogen consumption rate is normally too high to be economically acceptable. In other words, because of the low driving force and unavailability of a phase change (vaporization), an unacceptably high amount of nitrogen gas is required to implement the cooling operation without freezing. Furthermore, the cold gas mixture will lose its sensible heat very rapidly due to its low heat capacity, which makes it unacceptable for many heat transfer applications.
Other prior art systems have mixed spent cryogenic gas with the incoming cryogenic liquid to provide a resulting mixture of cryogenic cold gas. However, only the sensible heat component of the cryogenic cold gas contributes to refrigeration. As a result, the mixture loses its refrigeration ability very rapidly (as was the case when cryogenic liquid was mixed with room temperature gas, described above) and uniform cooling becomes very difficult. Also, the large volume of gas (caused by the combination of the evaporating liquid nitrogen and the added spent cryogenic gas) causes an excessive pressure drop and increases operating cost.